Sediment generated by interrill erosion is commonly assumed to be enriched in soil organic carbon (SOC) compared to the source soil. However, the reported SOC enrichment ratios (ER SOC) vary widely. It is also noteworthy that most studies reported that the ER SOC is greater than unity, while conservation of mass dictates that the ER SOC of sediment must be balanced over time by a decline of SOC in the source area material. Although the effects of crusting on SOC erosion have been recognized, a systematic study on complete crust formation and interrill SOC erosion has not been conducted so far. The aim of this study was to analyze the effect of prolonged crust formation and its variability on the ER SOC of sediment. Two silty loams were simultaneously exposed to a rainfall simulation for 6 h. The ER SOC in sediment from both soils increased at first, peaked around the point when steady-state runoff was achieved and declined afterwards. The results show that crusting plays a crucial role in the ER SOC development over time and, in particular, that the conservation of mass applies to the ER SOC of sediment as a consequence of crusting. A “constant” ER SOC of sediment is therefore possibly biased, leading to an overestimation of SOC erosion. The results illustrate that the potential off-site effects of selective interrill erosion require considering the crusting effects on sediment properties in the specific context of the interaction between soil management, rainfall and erosion.
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